composition- managing distant view of large objects
I think the main issue here is simply the viewpoint - in your scene the camera is actually about 54m above the surface of the lake; Suzanne is scales at 38m. As we (as humans) are used to viewing the world from about 1.5m above the 'floor', this throws out the perspective, giving the impression of a much smaller scene.

By simply moving the camera to about 2m from the surface of the lake you can get a much better perspective, making the mountains look much more distant :



Note that for added effect you could add a mist-pass in the compositor or blur based on distance for more realism.